High-speed phosphors and methods to prepare same

ABSTRACT

HIGH-SPEED PHOSPHOR IS ZINC OXIDE OR ZINC-CADMIUM OXIDE DOPED WITH ALUMINUM, GALLIUM, OR INDIUM. EACH ATOM OF DOPANT CONTRIBUTES APPROXIMATELY ONE FREE ELECTRON TO THE CONDUCTIVITY OF THE PHOSPHOR AND DUE TO THE ELIMINATION OF TRAPS, THE PHOSPHOR HAS VERY RAPID DECAY CHARACTERISTICS WHICH AARE ESTIMATED TO BE IN THE ORDER OF 10**-10 SECOND. IN THE PREFERRED MODE OF PREPARATION, THE PHOSPHOR IS FIRED IN HYDROGEN TO CAUSE THE FIRED MATERIAL, WHEN COOLED, TO DISPLAY SUBSTANTIALLY A MAXIMUM POSSIBLE VALUE OF ELECTRICAL CONDUCTIVITY. AMONG THE SPECIAL APPLICATIONS FOR SUCH PHOSPHOR ARE CATHODE-RAY DEVICES.

June 8, 1971 w. LEHMANN 3,583,928

HIGH-SPEED PHOSPHORS AND METHODS TO PREPARE SAME Original Filed Oct. 5, 1967 3,583,928 HIGH-SPEED PHOSPI-IORS AND METHODS T PREPARE SAME Willi Lehmann, Murrysville, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa. Original application Oct. 3, 1967, Ser. No. 672,493, now Patent No. 3,534,211. Divided and this application Mar. 25, 1970, Ser. No. 22,549

Int. Cl. C09k 1/10 US. Cl. 252301.6R 7 Claims ABSTRACT OF THE DISCLOSURE High-speed phosphor is zinc oxide or zinc-cadmium oxide doped with aluminum, gallium, or indium. Each atom of dopant contributes approximately one free electron to the conductivity of the phosphor and due to the elimination of traps, the phosphor has very rapid decay characteristics which are estimated to be in the order of 10- second, In the preferred mode of preparation, the phosphor is fired in hydrogen to cause the fired material, when cooled, to display substantially a maximum possible value of electrical conductivity. Among the special applications for such phosphor are cathode-ray devices.

CROSS REFERENCES TO RELATED APPLICATIONS This application is a division of application Ser. No. 672,493, filed Oct. 3, 1967, now Pat. No. 3,534,211.

In copending application Ser. No. 672,494, filed Oct. 3, 1967, now Pat. No. 3,534,210 by the present applicant is disclosed a high-speed phosphor and method, as well as a cathode-ray device, wherein the phosphor is cadmium sulfide or modified cadmium sulfide doped with aluminum, gallium or indium. While the matrix material of this cadmium sulfide phosphor is different from the present phosphor, the method of preparation is quite similar.

BACKGROUND OF THE INVENTION This invention broadly relates to high-speed phosphors and, more particularly, to high-speed phosphor composition, and the method for preparing same.

Phosphors which have a very rapid decay time are known as high-speed phosphors. Such materials have use in flying-spot scanners, control phosphors in color television tubes, scintillation counters, and experimental or manufacturing control phosphors such as might be used to determine a transit time and spread for photomultiplier tubes, to name a few of the applications.

Zinc oxide phosphor materials are well known in the art and respond to excitation by ultraviolet or cathode rays, for example, to emit primarily in two emission bands, a broad band in the green region of the visible spectrum, and the so-called edge-emission which for this phosphor is relatively narrow band in the near ultraviolet with a peak at about 390 nm. The green emission for such phosphors is normally obtained by firing zinc oxide under slightly reducing conditions. The so-called edge-emission of this material has been reported to be obtained by firing pure zinc oxide under oxidizing conditions, see Leverenz, An Introduction to Luminescence of Solids, published by Wiley, 1950, page 218. The green emission of the zinc oxide which is fired under the slightly reducing conditions is obtained under excitation with ultraviolet or cathode rays and the decay time is usually in the order of 1 l0 second. The foregoing ultraviolet edge-emission of pure zinc oxide which is fired under oxidizing conditions is not readily obtainable at room temperature unless the phosphor is excited by very high energy cathode rays and, even then, the re- United States Patent O Patented June 8, 1971 sponse of the phosphor is comparatively weak. The decay time of this phosphor is reported to be about 2x10- second by D. Hagan and K. Lertes in Z. Phys, 170, 367 (1962) and about 10* second by Leverenz at page 294 in Luminescence of Solids Wiley and Sons, New York (1950).

In explanation of the term edge emission, this can be defined as a phosphor emission which occurs at a wavelength which is only slightly longer than the long wavelength edge of the phosphor absorption spectrum.

The decay time is the primary factor in determining the so-called speed of the phosphor, since the speed of initial response of the phosphor to excitation is at least as fast as the speed at which the emission decays after excitation is removed, and is normally faster. The phosphor decay time as measured herein is that period of time required for the phosphor brightness to decay from a maximum value to a value which is 37% of the maximum value.

The term dope or dopant as used herein refers to a donor impurity which constitutes a lattice defect that is able to supply at least one free electron into the conduction band, or alternatively, an impurity that enhances the n-type conductivity of the material.

SUMMARY OF THE INVENTION It is the general object of the present invention to provide a phosphor material which has an extremely high speed.

It is another object to provide a method for preparing a phosphor material which has an extremely high-speed.

The foregoing objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing a phosphor composition having a matrix or zinc oxide or zinc-cadmium oxide wherein the' gram-atom ratio of zinc to cadmium is at least 9:1. This matrix is doped with predetermined amounts of aluminum, gallium or indium. Due to the conditions of preparation, the electrical conductivity of the phosphor is substantially at a maximum possible value, because of the substantial elimination of all traps in the phosphor. There is also provided a method for preparing the phosphor,

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS To prepare the present phosphor there is mixed together Group II-B metal compound of zinc or zinc plus cadmium wherein the gram-atom ratio of zinc to cadmium in the compound is at least 9:1, and Group III-A metal compound of aluminum, gallium, indium or mixtures thereof in such amount that the ratio of total gramatoms of Group III-A metal to Group Ill-B metal present in the mixture is from 0.0001 to 0.05. The compound of the foregoing Group II-B metal is the oxide or a compound which readily decomposes to the oxide on heating, and the compound of Group III-A metal is the oxide or a compound which readily decomposes to the oxide on heating.

The foregoing mixed compounds are fired in an oxidizing atmosphere at a temperature of from about 800 C. to about 1000 C. for at least about one half hour. Thereafter, the first fired material is refired either in a hydrogen atmosphere or inan atmosphere of zinc vapor at a temperature of from about 550 C. to about 700 C. for a sufficient period of time to cause the resulting fired material to display substantially that maximum possible value of electrical conductivity as is permitted by the amount of the aluminum, gallium, or indium in the phosphor.

EXAMPLE I As the first example, 3.75 grams of Al(NO -9H O is dissolved in 100 ml. of water, which provides an alu minum concentration of 10- gram-atom weight per ml. Thirty ml. of this solution are added to 81.4 grams (1 gram mole) of pure zinc oxide powder and the mixture thoroughly agitated as a slurry, and then dried. The dried powder is placed in a quartz container and fired in air at a temperature of from about 800 C. to 1000' C. for approximately one hour. The resulting first-fired material at best is very poorly luminescent. This first-fired material is refired in flowing hydrogen gas at a temperature of 600 C. to 700 C. for approximately 30 minutes. After cooling, the resulting phosphor displays an edge-emission having a peak at about 390 nm. when excited by 365 nm. ultraviolet, or by cathode rays. The phosphor has a speed of response, i.e., a decay time, which is so rapid that it could not be measured by available apparatus, which has limited to measuring a decay time of no less than about 5X10- second. It is estimated that the decay time of the phosphor is in the order of about 10* second.

EXAMPLE II 159 milligrams of dry aluminum oxide powder are mixed with 81.4 grams of dry zinc oxide powder by means of ball milling. Thereafter, the raw mix is prefired and then fired in flowing hydrogen gas in the manner as described in Example I. The resulting phosphor is identical to that achieved under Example I.

EXAMPLE III 700 milligrams of gallium metal are dissolved in hot nitric acid, the excess acid is boiled off, and the resulting solution then diluted to 100 ml. Thirty ml. of this solution (which corresponds to 10 gram atomic weight of gallium per ml.) are added to 81.4 grams of zinc oxide powder and the raw mix constituents are formed into a uniform slurry and then dried. Thereafter, the firing procedure is that which is described in foregoing Example I and the luminescence of the resulting phosphor is substantially identical to that reported under Example I.

EXAMPLE IV Aluminum is added either as a dry oxide or as a compound in solution, as per the foregoing examples, to pure zinc oxide. The mixture is prefired in accordance with the foregoing examples. The prefired material is then enclosed in a quartz tube to which is added approximately 100 to 200 milligrams of zinc metal. The tube is evacuated and sealed off. The entire tube is then fired at a temperature of approximately 700 C. for approximately 30 minutes so that the actual firing atmosphere for the phosphor raw mix constitutes an atmosphere of zinc vapor. After cooling, the phosphor is substantially identical to that reported under Example I.

In the foregoing examples, the concentrations of aluminum, gallium, indium, or any mixtures thereof, in compound form as specified, may vary between about 0.0001 to 0.05 gram atom per gram atom of total zinc plus cadmium in the phosphor matrix. The preferred dopant is aluminum or gallium or mixtures thereof present in amount of about 0.003 gram atom per gram atom of total zinc plus cadmium in the phosphor matrix.

While the foregoing examples all relate to finely divided or powder phosphor, the present phosphor can also be formed as a thin film. As an example, metallic zinc or zinc telluride is evaporated as a thin film onto a substrate. The evaporated and deposited zinc or zinc telluride is then converted to the oxide by heating in air.

4 The dopant is preferably codeposited as a metal with the matrix compound and then converted to the oxide. After conversion to the oxide, the film is heated in the hydrogen or zinc vapor atmosphere in the manner as described hereinbefore.

The final firing procedure in the hydrogen atmosphere or the atmosphere of zinc vapor, at a temperature of from about 550 C. to about 700 C. should be conducted for a suflicient period of time to cause the resulting fired material to display substantially that maximum possible value of electrical conductivity as is permitted by the amount of the dopant in the matrix. In explanation of this conductivity, the final firing under the specified conditions causes the total zinc plus cadmium in the matrix to exceed that amount which is required to form the stoichiometric oxide, and apparently this serves to fill all traps on the matrix, thereby accounting for the extremely fast decay since the traps account for the phosphoresence or afterglow. As a result, substantially each atom of the dopant in the phosphor contributes one free electron to the electrical conductivity of the phosphor. This is best illustrated by the results reported in the following Table 11:

TABLE I Influence of hydrogen firing atmosphere on electrical powder resistance 1 and on color or emission of photoluminescence at room temperature.

No dopant added 0.3% Go. added Ohms Color Ohms Emission Hydrogen atmos. 120, 000 Dead 5 Near UV with during retiring at violet tall.

N o refiring: 300, 000 Weak green 2, 500 Dead.

ranges. To illustrate, the calculated value of conductivity for non-particulate (i.e., solid) zinc oxide doped with the minimum specified amount of 0.0001 gram-atom per gramatom of zinc is about 2 mhos and the calculated value of conductivity for the maximum dopant amount of 0.05 gram-atom is about 10 mhos.

In the foregoing examples, the zinc can be replaced by cadmium oxide in such amount that the gram-atom ratio of zinc to cadmium is at least 9: 1. The zinc oxide in the raw mix could be replaced by a compound which read ily decomposes to the oxide on heating, such as the nitrate. Also, the doping compounds can be mixed in any relative proportions in the foregoing examples.

With specific reference to the form of the invention illustrated in the sole figure of the drawing, the numeral 10 illustrates a cathode-ray device which is generally of conventional constructional and comprises an evacuated envelope 12 includng a faceplate 14. An electron gun 16 is positioned within the neck portion of the envelope 12 and the device is provided with a conventional electron deflecting means, illustrated diagrammatically as deflecting coils 18 and 20. A phosphor screen 22 is positioned between the faceplate 14 and the electron gun 16 and this phosphor screen comprises the high-speed phosphor of the present invention. It should be clear that the tube 10 as illustrated is only shown in diagrammatic form and that the present high-speed phosphor can be used equally well in a color television tube as a control phosphor, in which case the present phosphor would be used in conjunction with conventional color TV tube phosphors, shadow mask, plural guns, etc.

1 Measured with 1.5 volts D.C. applied ltO a powder layer of 1 cm. area and 1 mm. thickness pressed between two brass electrodes.

The present phosphor also has application in flyingspot scanners and scintillation counters and because of the extremely fast decay time, there exist many other uses for this phosphor.

It will be recognized that the objects of the invention have been achieved by providing a phosphor composition which has an extremely fast rate of decay as well as a method for making such a phosphor.

I claim as my invention:

1. A high-speed phosphor composition having a matrix of zinc oxide or zinc-cadmium oxide wherein the gramatom ratio of zinc to cadmium is at least 9:1, said matrix being doped with at least one metal selected from the group consisting of aluminum, gallium, and indium, in amount of from 0.0001 to 0.05 gram-atom per gram-atom of total zinc plus cadmium in said matrix, said phosphor composition having a decy time of less than X second and an electrical conductivity within the range of 2 mhos to 10 mhos, said electrical conductivity being substantially at that maximum possible value as is permitted by the amount of said dopant in said phosphor.

2. The phosphor as specified in claim 1, wherein the total zinc plus cadmium in said matrix exceeds that amount required to form stoichiometric oxide.

3. The phosphor as specified in claim 1, wherein substantially each atom of said dopant in said phosphor contributes one free electron to the electrical conductivity of said phosphor.

4. The phosphor as specified in claim 1, wherein said dopant is aluminum or gallium or mixtures thereof present in amount of about 0.003 gram-atom per gram-atom of total zinc plus cadmium.

5. The method of preparing a high-speed phosphor having a decay time less than 5x10 second and an electrical conductivity within the range of 2 mhos to 10 mhos, which method comprises:

(a) mixing together (1) compound of the Group II-B metals zinc or zinc plus cadmium, wherein the gramatom ratio of zinc to cadmium in said compound is at least 9:1 and (2) compound of the Group III-A metals aluminum, gallium, indium, or mixtures thereof in such amount that the ratio of total gram-atoms of Group III-A metal to Group II-B metal present in said mixture is from 0.0001 to 0.05, said compound of Group II-B metal being the oxide or a compound which readily decomposes to the oxide on heating, and said compound of Group III-A metal being the oxide or a compound which readily decomposes to the oxide on heating;

(b) firing said mixed compounds in an oxidizing atmosphere at a temperature of from about 800 C. to about 1000 C. for at least about one-half hour; and

(c) refiring said fired material either in a hydrogen atmosphere or in an atmosphere of zinc vapor at a temperature of from about 550 C. to about 700 C. for a sufficient period of time to cause the resulting fired material, after cooling, to display substantially the maximum possible value of electrical conductivity.

6. The method as specified in claim 5, wherein said refiring is for a period of about 30 minutes.

7. The method as specified in claim 5, wherein said Group III-A metal compound is metal compound of aluminum or gallium or mixtures thereof in such amount that the ratio of total gram-atoms af Group III-A metal to Group II-B metal present in said mixture is about 0.003.

References Cited UNITED STATES PATENTS 2,529,711 11/1950 Smith 252301.6 2,585,461 2/1952 Hirsch 252301.6X 2,887,632 5/1959 Dalton 252301.6X 3,089,856 5/1963 Cyr et a1. 252-501X TOBIAS E. LEVOW, Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.'R. 252501, 518 

